When their brother Bob shows up at their Oklahoma hideout with the fugitive Belle Starr, outlaws Emmett and Grat Dalton greet him with concern. Bob explains that he rescued Belle from arrest after her husband Sam was killed in a shootout and insists on protecting her. Meanwhile, in the nearby town of Guthrie, Tom Bradfield, the owner of the Bird Cage saloon, is visited by Matt Towner of the Bankers Protective Association, who asks for his help in apprehending the Dalton gang. For $100,000, Tom agrees that Towner and Marshal Ripple can use his successful saloon to trap the outlaws. Later, at the Daltons' hideout, Emmett receives a message to meet itinerant trader Pete Bivins at his campsite. There, Pete tells Emmett that the Bird Cage's safe is full of cash and, after negotiating a cut of the loot for himself, suggests that the gang rob it that Saturday night. Emmett and Grat plan the robbery, excluding Ringo, the Indian who owns the hideout, and Ringo's friend Mac from the scheme. Bob, meanwhile, confesses his love to Belle, and although she resists him, she agrees to leave with him after he returns from the holdup. During the Daltons' absence, Mac and the sharpshooting Belle take some target practice, and their gunshots are heard by a passing Cherokee. Investigating, the Indian spots Belle and alerts Marshal Ripple and his posse. Belle sees the posse approaching, and she, Mac and Ringo flee in a hail of bullets. Later, at blacksmith Jim Clark's place, the trio concludes that the Daltons double-crossed them and, in revenge, plot to foil their Bird Cage holdup. Wearing the same type of bandana favored by the Daltons, Mac, Ringo and Belle, dressed in men's clothing, walk into the Bird Cage shortly before the Daltons are to arrive and begin robbing the place. When the unsuspecting Daltons show up, Ripple and his men open fire on them, interrupting the holdup. All of the outlaws escape unharmed, and Belle later convinces Mac and Ringo to form a new gang with her. In the meantime, Pete, who has been skillfully playing both sides, informs the Daltons about Belle's trick, but Bob continues to defend her. After Belle's gang executes a series of robberies, the press declares that she and the Daltons are involved in a crime combine. Furious, Emmett orders the still-enamored Bob to find Belle and determine where she stands. At the same time, Belle, Mac and Ringo, now sporting fancy attire, return to Guthrie and boldly enter the Bird Cage. At first, Tom does not recognize Belle, who is wearing a blonde wig and claims she is Lucy Winters, a widow from Montana. While playing blackjack with her, however, Tom realizes she is one of the robbers but says nothing. Instead, Tom suggests that Belle use her sizable blackjack winnings to go into partnership with him. Belle accepts the offer and hires Ringo as a look-out and Mac as a dealer. Soon after, Bob spies Mac outside the saloon and, at gunpoint, accuses him of stealing Belle. When Mac insists that Tom is the thief, Bob enters the saloon and sees Belle. Pete, meanwhile, informs Tom about Bob, and when confronted, Bob draws his guns on his unarmed rival. Before Bob can fire, Belle shoots his guns off his holster, and Bob then is arrested. Later, Belle instructs Ringo and Mac to break Bob out of jail before he reveals her identity. Bob remains faithful to Belle, however, and after he is freed, he and Belle finally clear up their misunderstandings. While Bob, Mac and Ringo ride out to the Daltons' new hideout, Tom confers with Towner about another plan to trap the Daltons. Tom proposes that they plant a newspaper story announcing that a million-dollar payoff is to take place at the bank. Belle sees the story and schemes to steal the money with the Daltons. Soon after, Tom invites Belle, whom he has nicknamed "Montana," for a romantic ride in the country. Tom speaks eloquently about the future of the West, and Belle finds herself falling in love. Later, Belle confesses her identity to Tom, who informs her that he has long known about her impersonation. Tom then convinces Belle to go to Mexico, where he will soon join her and marry her. As she is fleeing in Pete's wagon, however, Mac and Ringo waylay her and take her to the Daltons, who insist that she ride with them to Guthrie. Unaware of Tom's trap, Belle agrees on condition that Tom not be hurt. In town, meanwhile, Tom shocks Towner by declaring that he is going to marry Belle Starr. Just then, the Daltons, Mac, Ringo and Belle ride into town and, while Ripple's men lie in wait, rob the bank. Shots soon are fired, and during the ensuing gun battle, Belle is wounded. Tom manages to get her out of town, but after the remaining outlaws have been either captured or killed, Ripple sends his posse to find Belle. Unable to ride because of her injury, Belle prepares to give herself up, but promises Tom that she will marry him after she has served her time.

Although onscreen credits and reviews indicate that the picture was shot in color, the viewed print was in black and white. As noted by contemporary news items, Montana Belle was produced in 1948 by Fidelity Pictures, a company run by Howard Welsch. In Jul 1948, LAT announced that Fidelity had entered into a deal to make the story, which at the time was credited exclusively to M. Coates Webster, for release by Republic Pictures. Brian Donlevy and Ann Sheridan were mentioned as possible stars. Fidelity borrowed Jane Russell from RKO for the production.
News items and HR production charts indicate that Walter Colmes began as the film's associate producer, but was replaced by Robert Peters halfway through principal photography. According to a NYT news item, in Apr 1949, Welsch and Peters sold the picture, whose budget was estimated at $650,000, to RKO for $875,000. RKO head Howard Hughes then shelved the film for three and a half years. For more information about Belle Starr and other films inspired by her life, see entry for the 1941 Twentieth Century-Fox picture Belle Starr in AFI Catalog of Feature Films, 1941-50 . For more information about the Dalton Brothers, see entry for the 1945 Universal film The Daltons Ride Again in AFI Catalog of Feature Films, 1941-50 .

The American Film Institute is grateful to Sir Paul Getty KBE and the Sir Paul Getty KBE Estate for their dedication to the art of the moving image and their support for the AFI Catalog of Feature Films and without whose support AFI would not have been able to achieve this historical landmark in this epic scholarly endeavor.